A Trifecta of New Insights into Ovine Footrot for Infection Drivers, Immune Response, and Host-Pathogen Interactions

Abstract

Footrot is a polymicrobial infectious disease in sheep causing severe lameness, leading to one of the industry’s biggest welfare problems. The complex aetiology of footrot makes in-situ or in-vitro investigations difficult. Computational methods offer a solution to understanding the bacteria involved, how they may interact with the host and ultimately providing a way to identify targets for future hypotheses driven investigative work. Here we present the first combined global analysis of the bacterial community transcripts together with the host immune response in healthy and diseased ovine feet during a natural polymicrobial infection state using metatranscriptomics. The intra tissue and surface bacterial populations and the most abundant bacterial transcriptome were analysed, demonstrating footrot affected skin has a reduced diversity and increased abundances of, not only the causative bacteria Dichelobacter nodosus , but other species such as Mycoplasma fermentans and Porphyromonas asaccharolytica . Host transcriptomics reveals a suppression of biological processes relating to skin barrier function, vascular functions, and immunosurveillance in unhealthy interdigital skin, supported by histological findings that type I collagen (associated with scar tissue formation) is significantly increased in footrot affected interdigital skin comparted to outwardly healthy skin. Finally, we provide some interesting indications of host and pathogen interactions associated with virulence genes and the host spliceosome which could lead to the identification of future therapeutic targets. Impact Statement Lameness in sheep is a global welfare and economic concern and footrot is the leading cause of lameness, affecting up to 70% of flocks in the U.K. Current methods for control of this disease are labour intensive and account for approximately 65% of antibiotic use in sheep farming, whilst preventative vaccines suffer from poor efficacy due to antigen competition. Our limited understanding of cofounders, such as strain variation and polymicrobial nature of infection mean new efficacious, affordable and scalable control measures are not receiving much attention. Here we examine the surface and intracellular bacterial populations and propose potential interactions with the host. Identification of these key bacterial species involved in the initiation and progression of disease and the host immune mechanisms could help form the basis of new therapies.

Lesion Material From Treponema-Associated Hoof Disease of Wild Elk Induces Disease Pathology in the Sheep Digital Dermatitis Model

“…T. medium, T. pedis, and T. phagedenis have consistently been found within TAHD lesions (2, 3) as well as within bovine DD lesions (2,3,15,31,(39)(40)(41)(42)(43)(44)(45)(46)(47). Using species specific primers, one or more of these species were identified on inoculated foot swabs in at least one sampling timepoint (Table 2).…”

Natural Mycoplasma Infection Reduces Expression of Pro-Inflammatory Cytokines in Response to Ovine Footrot Pathogens

“…It is a significant welfare issue and has an enormous financial impact, which often limits interventions and treatments used to eradicate the infection from a flock which in turn reduces productivity [2]. With 90% of flocks in the UK affected, a mean prevalence of 4.5% [1] and the increased pressure to reduce antibiotic use [3], there is an ever present drive to broaden our understanding of the disease and use novel experimental approaches [4][5][6][7] in the effort to improve the situation. Despite knowing the main causative bacterium, Dichelobacter nodosus (D. nodosus) [8], risk factors involved, such as an impaired skin barrier and poor environmental conditions [9], and having vaccines available, little progress has been made to reduce the incidence.…”

“…Recent publications have helped the understanding the microbial communities present on the surface of the interdigital sheep skin [10,11] and deeper within the interdigital skin layers [5] by shedding light on the complexity of this disease and the different bacterial species potentially involved in the initiation, progression, and exacerbation of the disease [7]. The bacterial species found deeper within the interdigital skin structure and associated with footrot were D. nodosus, Mycoplasma fermentans, (M. fermentans), and Porphyromonas asaccharolytica (P. asaccharolytica).…”

“…Once the basement membrane is overcome or even damaged, fibroblasts, interspersed in the extracellular matrix of the underlying dermis, are activated and release pro-inflammatory cytokines, chemokines and prostanoids [13,14]. However, a comprehensive transcriptomic assessment of ovine interdigital skin identified the suppression of cytokines, chemokines, metalloproteases and their regulators in footrot affected tissue, suggesting a local dampening of wound healing and immune cell recruitment [7]. Interdigital skin, comprised of epidermis and dermis, is at the forefront of the primary immune response against footrot.…”

“…To clarify the role of the cytokines and chemokines that have been identified in earlier an explant model [16], expression and release of IL-1β and CXCL8 were measured after stimulation with E. coli lipopolysaccharide (LPS), heat-inactivated D. nodosus, Ultraviolet lightinactivated D. nodosus, formalin-fixed D. nodosus or heat-inactivated F. necrophorum. Due to the identification of a natural infection of ovine digital skin with Mycoplasma fermentans (M. fermentans) from metatranscriptomic data [7] these experiments were carried out with and without its presence in the ovine skin cell cultures.…”